It is required to further reduce components built in an electronic device such as a mobile device, in size and weight. For example, it is required that a filter and a duplexer which are used for the mobile device be small in size, that the frequency characteristics thereof be precisely adjusted, and that the insertion loss thereof be small. As a filter for satisfying the above requirements, a piezoelectric filter using a piezoelectric resonator is known.
FIG. 11 is a cross-sectional view of a conventional piezoelectric resonator.
In FIG. 11, a first piezoelectric resonator 1101 and a second piezoelectric resonator 1102 are formed on the same substrate 1103. The first piezoelectric resonator 1101 is formed on a first cavity 1104 provided in the substrate 1103 and the second piezoelectric resonator 1102 is formed on a second cavity 1105 provided in the substrate 1103, in such a manner that a lower electrode layer 1106, a piezoelectric layer 1107, and an upper electrode layer 1108 are positioned in order starting from the bottom so as to cover the first cavity 1104 and the second cavity 1105. The upper electrode layer 1108 corresponding to the first piezoelectric resonator 1101 is processed to be thinner than the upper electrode layer 1108 corresponding to the second piezoelectric resonator 1102.
The first piezoelectric resonator 1101 and the second piezoelectric resonator 1102 function as a resonator by each applying an electrical field between the upper electrode layer 1108 and the lower electrode layer 1106, thereby polarizing and distorting the piezoelectric layer 1107 so as to produce mechanical resonance, and by electrically extracting the mechanical resonance. The resonance frequencies of the first piezoelectric resonator 1101 and the second piezoelectric resonator 1102 are mainly determined based on the film thicknesses and the mass loading effects of vibration sections including the upper electrode layer 1108, the piezoelectric layer 1107, and the lower electrode layer 1106. Thus, it is possible to set the resonance frequency of the first piezoelectric resonator 1101 to be higher than the resonance frequency of the second piezoelectric resonator 1102 by making the upper electrode layer 1108 corresponding to the first piezoelectric resonator 1101 thinner than the upper electrode layer 1108 corresponding to the second piezoelectric resonator 1102, as shown in FIG. 11.
Alternatively, a layer thickness adjustment method for setting the resonance frequency of the first piezoelectric resonator 1101 to be high may employ, instead of making the whole upper electrode layer 1108 corresponding to the first piezoelectric resonator 1101 thin as shown in FIG. 11, a method of making thin some portion thereof as a whole (FIG. 12) or a method of dividing a portion to be made thin into a plurality of portions (see Patent Document 1).
Note that in order to vary the thickness of a piezoelectric resonator by differentiating the laminate thicknesses of a layer as described above, ordinarily, a mask is designed to correspond to a portion to be removed or a portion to be left, so as to use a photolithography technique (see Patent Document 2). The photolithography technique as used herein refers to a technique for sequentially performing a process of resist coating, exposure using the mask, development, etching, and resist removal.
Patent Document 1: Published Japanese Translation of a PCT Application No. 2002-515667
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-359534
In the conventional structures shown in FIGS. 11 and 12, however, in order to form three or more piezoelectric resonators having resonance frequencies different from one another on the same substrate, it is required to design a plurality of masks different from one another for frequency adjustment and to perform the photolithography process twice or more. This is disadvantageous for manufacturing a device at lower cost.
Thus, the objects of the present invention are to manufacture a device at low cost and obtain a high yield of a device, by simplifying a process and reducing consumption of masks, resist and the like, so as to realize a high-Q piezoelectric resonator and consequently provide a low-loss piezoelectric filter.